Alkali metals (lithium, sodium and potassium) form a large variety of hydrides: simple hydrides (LiH, NaH, KH) and complex hydrides with other elements, for example, boron or aluminum. Many of these compounds are commonly used in various processes of organic chemistry, acting as reducing agents.
Because of the high reactivity of alkali metals, simple hydrides of Li, Na and K are produced in a direct reaction between molten alkali metal and hydrogen at very high pressures and temperatures. For complex hydrides, in each case a special process of fabrication has been developed.
Lithium aluminum hydride (LiAlH.sub.4) was discovered about four decades ago [1] and since then it has become the most common reducing agent in many chemical reactions. Sodium aluminum hydride (NaAlH.sub.4) was first synthesized in the early sixties [2], but it has never been used as widely, because of its more difficult fabrication as compared with LiAlH.sub.4. Another complex hydride: sodium boron hydride (NaBH.sub.4) [3] is also known as having good reduction ability in many organo-chemical reactions [4].
In the search for even better reducing agents, many other complex hydrides based on lithium, sodium or potassium have been synthesized as, for example, LiBH.sub.4, LiAl.sub.4 H.sub.13, LiAl.sub.2 H.sub.7, Li.sub.3 AlH.sub.6, KAlH.sub.4, KBH.sub.4, K(Al(BH.sub.4).sub.4).
In all cases the synthesis of these complex hydrides is performed through a chemical reaction under special conditions. For example, commercial fabrication of LiAlH.sub.4 involves the reaction of LiH with AlCl.sub.3 in diethyl ether [5]. In the early sixties Ashby and co-workers developed an alternative route of production of complex metal hydrides by direct synthesis [2, 6, 7]. This method can be applied to the production of LiAlH.sub.4, NaAlH.sub.4, KAlH.sub.4 and CsAlH.sub.4. According to Ashby, synthesis of, for instance, NaAlH.sub.4 can be performed in the following way: "one charges either the alkali metal or its hydride to an autoclave with activated aluminum powder in a solvent such as tetrahydrofuran. The mixture then is subjected to a pressure of 2000 p.s.i. (about 140 bar) with hydrogen and heated to 150.degree. C. for several hours. After the absorption is complete the mixture is cooled and the complex aluminum hydride is separated from excess of aluminum by filtration. NaAlH.sub.4 can be isolated by addition of a hydrocarbon such as toluene to the tetrahydrofuran solution, followed by vacuum distillation of the tetrahydrofuran" [2].
Another complex alkali metal hydride, Na.sub.3 AlH.sub.6, was primarily fabricated by Zakharin et al [8] in the reaction of NaH and NaAlH.sub.4 at 160.degree. C. in heptane. However, due to insolubility of the product in all solvents tested, it could not be purified. In response to the drawbacks of the above method Ashby et al proposed again a direct method for the synthesis of Na.sub.3 AlH.sub.6 hydride [2, 7]. According to the direct method, Na.sub.3 AlH.sub.6 can be synthesized by the following reaction: EQU 3Na+Al+3H.sub.2.fwdarw.Na.sub.3 AlH.sub.6
This reaction should be performed in toluene at 165.degree. C. and at 5000 p.s.i. (about 350 bar) of hydrogen pressure.
Synthesis of Li.sub.3 AlH.sub.6 was first discovered by Ehrlich et al [9,10]. Later on, Mayet and co-workers [11, 12, 13, 14] developed another method of fabrication of Li.sub.3 AlH.sub.6, which provided better reproducibility and higher purity of the hydride. In this method a solution of LiAlH.sub.4 in ether is added drop by drop into a suspension of LiH in toluene heated up to the temperature of 50.degree. C. The mixture is kept for several hours at 50.degree. C. to eliminate ether and is subsequently heated up to 95.degree. C. to complete the reaction. Hot catalyst: Al(C.sub.2 H.sub.5).sub.3 in toluene or etherate of triethylaluminum, is added during the first step of the process.
All the above methods for the production of complex alkali metal hydrides suffer from many drawbacks, i.e. the need to use solvents or dispersing liquids (hydrocarbons) with activators, multi-step character and relatively poor yield and reproducibility.
In order to overcome these problems, another method of fabrication of complex alkali metal hydrides was developed more recently by Dymova and co-workers [15, 16, 17, 18]. In this method the solvents were eliminated, but instead a reaction at a temperature above the melting point of the alkali metal was proposed. The reaction of molten alkali metals (Li, Na, K, Cs) with aluminum was performed at a temperatures of 200-400.degree. C. and at hydrogen pressure of 100-400 bar.
In conclusion, all the previous methods of fabrication of complex alkali metal hydrides have three main disadvantages:
i) inconvenience of the use of toxic and flammable solvents such as toluene and tetrahydrofuran; PA1 ii) very high hydrogen pressures (100-400 bar); and PA1 iii) high temperatures (100.degree. C.-400.degree. C.).
Alkali-metal-based complex hydrides were developed with a clear purpose to serve as reducing agents in chemical reactions, mainly in organic chemistry. However, other applications of these hydrides have also been considered. Most of these hydrides undergo decomposition at high temperatures. The decomposition releases hydrogen and therefore alkali-metal hydrides can be used in some cases as an immediate source of hydrogen [19]. For example, LiAlH.sub.4 decomposes when heated up to a temperature of 125.degree. C. and releases gaseous hydrogen. This phenomenon has been exploited in equipment for hydrogen storage. It should be stressed however that these applications use alkali metal hydrides for a single, irreversible hydrogen release. There is no way to reverse the dehydrogenation reaction in these prior systems, without repeating the whole chemical procedure used in the production of the hydride, which obviously cannot be accomplished inside the hydrogen storage tank.
The present invention seeks to develop materials which can be used as a reversible source of hydrogen, i.e., which can be reversibly hydrided and dehydrided in subsequent cycles of hydrogen admission and evacuation, without any other treatment. The only prior method of yielding reversibility of hydrogenation in alkali-metal-based hydrides was reported in a recently published paper of Bogdanovic and Schwickardi [20]. The authors studied traditional alkali metal aluminum hydrides (NaAlH.sub.4 and Na.sub.3 AlH.sub.4) and state that "the reverse reaction has not been accomplished" until their method of doping with special Ti-based catalysts was developed [21]. The authors fabricated alkali metal hydrides in a conventional way (following the process described by Zakharin [8]). For example, Na.sub.3 AlH was prepared from NaAlH.sub.4 and NaH in heptane under hydrogen. The suspended reagents were intensively stirred at 162.degree. C. for 72 h under a hydrogen pressure of 140 bar. The reversible hydrogenation was achieved when the materials were treated with 2 mol % of .beta.-TiCl.sub.3 % in ether or with 3 mol % Ti(OBu).sub.4 in ether.
Although the results showed significant improvement of the hydride performance as compared to the undoped materials, the authors indicate that the kinetics of the process were still unsatisfactorily slow (for example, one absorption cycle took five to twenty or even 100 hours) and suffered from cyclic instability.